REV. 0

–18–

AD607

As noted earlier, the gain in dB is reduced linearly with the volt-

(1.5 V) and a supply voltage of 3 V is used. Figure 44 shows

how these vary when GREF is connected to a 1.23 V reference.

(7.5dB)

(1.5dB)

01V

2V

0.4V

1.8V

2.2V

(67.5dB)

(21.5dB)

IF GAIN

MIXER GAIN

90dB

80dB

70dB

60dB

50dB

40dB

30dB

20dB

10dB

0dB

NORMAL OPERATING RANGE

Figure 43. Gain Distribution for GREF = 1.5 V

(7.5dB)

(1.5dB)

01V

2V

(67.5dB)

(21.5dB)

IF GAIN

MIXER GAIN

90dB

80dB

70dB

60dB

50dB

40dB

30dB

20dB

10dB

0dB

0.328V

1.64V

NORMAL OPERATING RANGE

Figure 44. Gain Distribution for GREF = 1.23 V

Using the Internal AGC Detector

The AD607 includes a detector cell at the output of the IF am-

plifier that allows it to provide its own AGC and output-leveling

function in receiver applications where DSP support is not

needed. It is only necessary to connect a filter capacitor between

the GAIN pin and ground to invoke this feature. The voltage

appearing on this pin may then be used as an RSSI output, with

the scaling discussed earlier; note particularly that the voltage on

GREF continues to determine this scaling.

Figure 45 shows a simplified schematic of the detector. Transis-

eral,

≈ V

LAST IF STAGE

IFOP

1.5V

IF OUTPUT

Q1

Q2

77µA

30µA

˜

4.5µA

ZERO

1.5V + 316mV

GAIN

TO INTERNAL

GAIN CONTROL

CAGC

(EXT)

4.5µA

(INT)

COMM

AVERAGE OF IC2 IS

FORCED TO 4.5µA BY

INTEGRATION IN CAGC

Figure 45. Simplified Schematic of AGC Detector

Acting against this is an internally generated 4.5

µA pull-down

current, which operates to within a few millivolts of ground. As

ducing the amplitude of the IF output and reducing the ampli-

tude of the current spike in Q2; eventually a point is reached

where its average collector current is balanced by the pull-down

current, and the charging ceases. It will be apparent that the

loop filter is essentially a perfect integrator.

This simple system can be used because the input impedance of

the gain-control system, also internally tied to the GAIN/RSSI

pin, is several megohms, and its bias current is small. The volt-

heavily loaded, a buffer amplifier must be used.

Note that, unlike a post-demodulation AGC detector (via DSP),

this scheme responds to signal plus noise. Thus, when operating

at high gains, the AGC loop will “see” a substantial output at

the IFOP node, even though a filter may be added by the user

between the pins IFOP and DMIP. This will trick the loop into

lowering the gain until the composite output signal (IF plus

noise) reaches the reference level and satisfies the average-

current requirement. In these circumstances, the wanted signal

will be smaller than expected. Thus, the internal AGC system

will result in a slight compression of the demodulated output for

very small signal levels.

AGC Discharge Time

The discharge current is approximately 4.5

µA; thus, to restore

gain in the event of a rapid drop-out requires a time of

T = C

× V

80 dB gain change corresponds to 1.6 V, the discharge time is

355

µs. Note, however, that when GREF is tied to a different

value, the scaling changes. For GREF = 1.23 V, the scale factor

is 16.4 mV/dB, 80 dB corresponds to a 1.312 V change, and the

discharge time decreases to 290

µs.

works out to T = C

× P × 44,000, where P is the change in input

power, expressed in dB. Thus, using C = 1 nF, checking the

time needed for 80 dB we get T = 355

µs. For the case where

the scaling is 16.4 mV/dB, T = C

× P × 36,000.

The AD607’s AGC detector delivers only one brief charging

pulse per cycle of the IF. At a 10.7 MHz IF, for example, this is

every 93 ns. When the AGC system is in equilibrium, this pulse